Ageing of permanent magnet devices at the ESRF

This paper reviews two typical cases of permanent magnet demagnetization which have occurred during the seven years of X-ray beam delivery at the ESRF. The first case occurred during the ESRF commissioning and was due to a miss-steered electron beam. The second case was seen over a long period and was due to the routinely produced radiation (i.e. X-rays, associated with the parasitic radiation). The first process is illustrated with an experiment where various grades of permanent magnets have been voluntarily irradiated with an electron beam in order to study their susceptibility.